1. Field of the Invention
This invention relates to a device for detecting lamp failure in automobiles such as burn-out of lamps or short-circuit, and more particularly to such a device which utilizes the change of voltage depending on the electric current change caused by lamp failure as an object to be detected.
2. Description of the Prior Art
Japanese patent application No. 54-51041 discloses a lamp failure detecting device of the type, which employs a means for detecting the change of potential difference across ends of a current-detecting resistance connected in series to a plurality of lamps connected in parallel in the occurrence of electric current change caused by failure of one or a plurality of lamps. FIG. 6 illustrates a construction of such a prior art device which is used for detecting failure of turn indicator lamps of automobiles. In FIG. 6, numeral 1 designates a storage battery and numeral 2 a plurality of right side turn indicator lamps connected in parallel. The parallel circuit comprising the lamps 2 is connected to the battery 1 through a turn indicator switch 3, a current-detecting resistance 4, an ignition switch S.sub.1 and a fuse F. Voltage divider circuits 9 and 10 comprising resistances 5, 6 and 7, 8 respectively are connected to respective ends A and B of the current-detecting resistance 4. Voltage-dividing points A.sub.1 and B.sub.1 of the voltage divider circuits 9 and 10 are connected to a non-inversible input terminal (+) and an inversible input terminal (-) of a comparator 11 respectively. When failure occurs to one of the lamps, the value of current flowing through the current-dividing resistance 4 is decreased and voltage is raised at a connection B of the resistance 4, that is, at the voltage-dividing point B.sub.1 of the voltage divider circuit 10. Output of the comparator 11 is inverted, thereby detecting failure of the lamp.
As shown in FIG. 7, however, each lamp 2 shows a non-linearity that an increase ratio of the current I flowing through each lamp 2 is decreased as power-supply voltage V.sub.P increases because the resistance at each lamp 2 rises as the power-supply voltage increases with the temperature of a filament rising. When the power-supply voltage V.sub.P increases, voltage V.sub.0 at the voltage-dividing point A.sub.1 of the voltage divider circuit 9 increases linearly in accordance with the increase of the power-supply voltage V.sub.P as shown in FIG. 8. Voltage at the voltage-dividing point B.sub.1, however, increases non-linearly in the normal state where no lamp failure occurs and in the case where failure occurs to one of the lamps, as shown by V.sub.1 and V.sub.2 in FIG. 8 respectively. In order that failure of the lamps 2 is detected by the comparator 11, voltages V.sub.1 and V.sub.2 at the voltage-dividing point B.sub.1 in the normal state and in the case where failure occurs to one of the lamps respectively, necessitate taking the values below and above the voltage V.sub.0 at the voltage-dividing point A.sub.1 respectively as shown in FIG. 8. A range L of the power-supply voltage capable of correctly detecting lamp failure (a lamp failure detectable range) is restricted to a narrow region L.sub.1 between intersections P.sub.1 and P.sub.2 of a line denoting the voltage V.sub.0 at the voltage-dividing point A.sub.1 and two curved lines denoting respective voltages V.sub.1 and V.sub.2 at the voltage-dividing point B.sub.1 in the normal state and in the case where failure occurs to one of the lamps. As a result, when the power-supply voltage is increased beyond the lamp failure detectble range, correct detection of lamp failure cannot be expected. This means that an error detection occurs in the case of automobile in which the power-supply voltage is not constant.
In a device disclosed by Japanese patent application No. 51-156981, an operational circuit is provided for compensating for non-linearity of the current flowing through the lamps, thereby preventing the error detection and enlarging the lamp failure detectable range. However, this construction necessitates an expensive circuit for compensating for non-linearity of the current flowing through the lamps, which raises the production cost of the device.